It is known from Mukherjee at al. (Mukherjee, B. Viswanath B. and Ravishankar, N. (2010). Functional nanoporous structures by partial sintering of nanorod assemblies, Journal of Physics D: Applied Physics, Volume 43, 455301 (6pp)) that it is possible to sinter nanostructures and generate defined pores by coordinated temperature controls. The pores were generated by partial melting and rounding by subsequent sintering of nanorod edges, which as a result of anisotropic heat distribution were hotter than the remaining nanorod structures. This approach theoretically allows functional surfaces to be preserved on portions of nanorods that were not melted and sintered, The melting point of a nanoparticle differs from the melting point of the native material in bulk. As a result of the higher surface-to-volume ratio, the melting temperature is lowered drastically as a function of the particle size. This connection was shown by Buffat and Borel for nanoparticles made of gold (Buffat, O., Borel, J.-P. (1976). Size effect on the melting temperature of gold particle. Phys. Rev, A, 13, 2287-2298). It was demonstrated that particles having a diameter smaller than 100 nm begin to melt at lower temperatures than would be expected based on the melting point of pure gold in bulk at 1337 K. The melting point of particles smaller than 5 nm is below 350 K. Similar results were also shown by Allen at all for other materials, such as indium, tin, bismuth and lead, (Allen, G. L., Bayles, R. A., Gile, W. W., Jesser, W A (1986). Small particle melting of pure metals. Thin Solid Films, 44, 297-308). The melting point is also influenced by the stabilizing shell of the nanoparticles or solvents in which the particles are dissolved (Liang, L. H., Shen, C. M., Du, S. X., Liu, W. M., Xie, X. C., Gao, H J. (2004). Increase in thermal stability induced by organic coatings on nanoparticles. Physical Review 8, 70, 205419 (5 pp)).
It is known from Amert et al. (Amert, A. K., Oh, D. -H., Kim, N. -S. (2010). A Simulation and experimental study on packing of nanoinks to attain better conductivity. Journal of Applied Physics, 108, 102806 (5pp)) to consecutively sinter two different materials so as to generate a particularly densely packed structure on a substrate.
Various methods are known for producing nanoporous thin films having different layer thicknesses in the range of several nanometers to several micrometers, and pore sizes from one nanometer to hundreds of nanometers. The special characteristic of these materials is the large surface-to-volume ratio, A multitude of pores increases the available active or functional surface per unit of volume compared to planar thin films.
This phenomenon can be an advantage in various areas. One known example is immunosensors for infectious diseases. These sensors take advantage of specific binding reactions between antibodies and antigens. Antigens bound to the surface can specifically bind antibodies present in an analyte, such as in the blood or in saliva, and vice versa. The signals generated by the binding reactions can be read out, for example optically by adsorption and the refractive index, or electrochemically, such as by way of a change in impedance, or by Faraday or capacitive effects. In this way, the concentration of antibodies is determined. Nanoporous structures serving as electrodes on immunochemical sensors can produce signal amplification, and thus render the sensors more sensitive, at the same sensor volume or lateral sensor surface. The larger the electrode surface, the more antibodies are able to react with antigens. In addition, non-specific binding signals of other molecules that cause noise in the system and worsen the detection threshold can be prevented by defining the sizes of the pores. Nanoporous materials can additionally be used in lab-on-a-chip systems. Due to the large surface of the active sensor electrodes, the size of the system can be reduced and the packing density of these sensors on the chip can be increased. This contributes to the ability to implement lab-on-a-chip systems comprising multiple sensor elements for various analytes.
Other applications of nanoporous materials can be found in energy storage, catalysis, membrane production, tissue engineering, photonics, adsorption, separation and drug delivery. In all these fields, the large active surface or certain pore sizes help achieve required functionality.
The methods for producing nanoporous thin films are carried out in multiple steps using chemical, mechanical or electrochemical method steps. The disadvantage is that a plurality of chemical reagents and clean room techniques are employed in the process. Anodization is one known method for producing nanoporous layers. A metal layer, for example made of aluminum, is electrochemically oxidized in an acid solution. Nanostructured films are formed as a result of self-organization.
Nanolithography is another method. Here, structures are implemented in four steps. First, a thin film of the desired material is applied to a substrate by way of chemical vapor deposition (CVD), physical vapor deposition (PVD), laser pulsed deposition (LPD) or an equivalent method. Thereafter, a thin layer of photoresist is applied, The photoresist is structured using various lithographic methods. Known methods are VIS lithography, UV lithography or EUV lithography, electron beam lithography and interference lithography. The non-cured portion of the photoresist, which is to say the cross-polymerized portion in the case of a positive method, or the softened portion of the photoresist, this being the portion having broken chemical bonds, in the case of a negative method, is removed by washing. Thereafter, the pores are transferred into the material using dry or wet etching processes. At the end of the process, the etching mask is removed.
The disadvantage is that the use of strong chemical reagents always necessitates thorough cleaning of the thin films that are produced so as to remove residues of toxic substances. This makes manufacturing with these methods complex. Production must always be carried out in multiple steps, and further requires clean room technologies for all lithographic processes. This drives up the cost and manufacturing time for the layers produced and the sensors. It is also disadvantageous that many of the described methods can only be applied to certain materials since they are dependent on certain chemical properties of the substrates.